Physics 102
		   Homework set 8 Solutions

Chapter 26:

Exercises:

16: On average, light travels slower in glass because it is repeatedly
absorbed and reemitted by the molecules in the glass.  In between the
molecules, it travels at the same speed (300 million m/s).  Thus, when
it emerges from the glass it travels at the same speed that it had
before it entered it.  Unlike the bullet, the light  does not lose
energy in the glass.

22: The dark sunglasses are dark because they absorb light.  Therefore
they also absorb the energy of that light and they will heat up
faster.

26: a) You are on the moon during a lunar eclipse (as seen from the
Earth).  The shadow of the Earth falls on the moon.  You see the Earth
pass between you and the sun.  It gets dark.  You are observing an
eclipse of the sun.

b) You are on the moon during during a solar eclipse (as seen from the
Earth).  The shadow of the moon falls on the Earth.  You see a partial
eclipse of the Earth.

28: The light from the moon is white and therefore contains all
colors.  We cannot see the colors because there is not enough light
for our cone cells to detect (they see colors) so we have to use our
rod cells (which only see black and white).

Problems:

4: It is 3.84 * 10^8 m from the Earth to the moon.  The laser light
travels twice this distance or 7.68 * 10^8 m.  It takes a time
t = distance / velocity = 7.68 * 10^8 m / 3 * 10^8 m/s = 2.56 s

6: frequency = speed / wavelength = 3 * 10^8 m/s / 589 * 10^(-9) m
             = 5.1 * 10^14 Hz


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Chapter 27:

Exercises:

10: White paper reflects all colors.  Thus, whatever color of light
shines on it, it reflects that color.

18: The water absorbs some of the red light so your feet don't look as
red.

20: 
a) Yellow = green + red so yellow + blue = red+green + blue = white
b) You have to add red+blue to green to get white.  Thus
    green + magenta = white
c) red+blue + red+green + green+blue = white

26: The stars are still there during the day.  We can't see them then
because the atmosphere is too bright.  The sun is so bright and the
atmosphere scatters blue light, so the sun lights up the atmosphere,
overwhelming the feeble light from the stars.

28: Yes, you can see the stars from the moon during the day.  There is
no atmosphere on the moon so there is no light coming from the
atmosphere.  As long as you don't look at the sun, you can see the
stars.

34: Our sunsets are red because our atmosphere scatters blue light
(and a little green light too).  At sunset, when the light from the
sun has to travel through a lot of atmosphere, all the blue and green
light scatters away, leaving the red light.

If the sky was orange, it means that it scatters red and yellow
light.  This means that at sunset, all the red and yellow light would
be scattered, leaving only blue (or maybe blue plus a little green).
(Blue, blue+green, and cyan are all acceptable answers.)


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Estimation:

The dynamic range of the human eye: 
a) My pupils are 3 mm in diameter indoors, so they would be about 1-2
mm in diameter in bright sunlight.  This gives me an area for one
pupil of about 1 mm^2 or 10^(-6) m^2.  The solar power entering my eye
is 1000 W/m^2 * 10^(-6) m^2 = 10^(-3) W.  In 1/10 s, this gives an energy
= power * time = 10^(-4) J.

b) Seven photons have an energy of 7 * 2.5*10^(-18) J = 2 * 10^(-17)J

Thus, the human eye has a dynamic range of 10^(-4) J / 10^(-17) J 
= 10^13!  It can adapt over 13 orders of magnitude!  That's far
better than any optical devices built by people.